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Regulation of VEGF
Introduction Vascular endothelial growth factor (VEGF) is a protein that promotes angiogenesis in many parts of the body . Proper regulation of VEGF is extremely important since overexpression leads to unregulated and increased blood vessel growth 1,2,3,4,. This has been linked to cancers and many other human diseases like arthritis, macular degeneration, and psoriasis 1. In this article VEGF will be discussed relating to blindness, wound repair, cancer, and stroke. Background VEGF was first identified in 1982 by Seneger et al. in hamsters, but they called it vascular permeability factor 5. In 1989, Leung et al. isolated and cloned the cDNA of VEGF from bovine. They were the first to classify it as an angiogenic stimulator 6. VEGF permeabilizes the surrounding epithelial tissue to allow for neovasculargenesis 1. The VEGF family consists of VEGF A, involved in wound repair 1, VEGF B, VEGF C, VEGF D, VEGF E, VEGF F, and placental growth factors 2. There are also three tyrosine kinase receptors VEGFR-1, VEGFR-2, and VEGFR-3, and two other non-enyzymatic factors 1,2. For the purposes of this article we are going to focus on VEGF A and VEGFR-1 and VEGFR-2. Mechanism VEGF A, is a heterodimeric, heparin-binding protein and binds to both VEGFR-1 and VEGFR-2. The VEGF receptors are tyrosine kinase receptors that have a extracelluar domain consisting of 7 immunoglobulin like binding regions, a single pass transmemebrane domain, and an intracellular tyrosine kinase domain. VEGF (assume VEGF A unless specified) binds to both receptors with a higher affinity for VEGFR-1. VEGFR-1 acts as a regulator of VEGF because the tyrosine kinase domain is inactive. Once VEGF binds to VEGFR-2 the pathway gets turned on; this inhibits apoptosis and promotes proliferation and cell migration (Figure 1). VEGF can also bind to neuropilins, the non-enzymatic factors, that act as positive regulators or coreceptors increasing the activity of VEGFR 1 . Regulation VEGF expression is increased during wound healing, largely because of hypoxia 1. The receptors expression is also enhanced during low oxygen events. Strokes also increase VEGF expression in the brain for the same reason 2. During times of decreased oxygen blood vessel growth is necessary to provide oxygen to the damaged tissues and promote healing 1,2. Transcriptional Transcription can be enhanced under hypoxic conditions with a 5' enhancer region that encodes a hypoxia inducible factor (HIF) binding site 3,4,7. The figure to the right is a schematic, from Levy et al. showing HIF binding. HIF is regulated by the von Hippel-Lindau tumor suppressor protein (VHL)7. VHL is a complex involved in the ubiquintation proteins under normal oxygen levels7,8. When oxygen levels are fine, HIF-a binds to VHL and gets degraded 7, but under hypoxic conditions the VHL complex is ubiquinated, thus degraded 8. This allows HIF-a and HIF-b to bind VEGF-A and transcribe mRNA 7. Post-transciptional VEGF mRNA contains both stabilizing and destabilizing factors, with a half-life of 30-45 minutes under normal conditions. The destabilizing regions are known as adenylate-urdylate-rich elements (AREs). Under hypoxic conditions HIF can bind the AREs and stabilize the mRNA so that it can be translates 3,4,7. The mRNA can also be alternatively spliced. The regulations of this splicing is still unknown but this allows for the existence of inactive isoforms. Also leading to different isoforms are the two poly-adenylation sites. The function of this is being actively researched as it could be an interesting level of regulation 4. Translational On top of transcription being highly regulated, translation is also tightly controlled. Two methods that also lead to various isoforms are the use of multiple start codons, AUG and CUG, as well as and upstream open reading frame. Other proto-oncogenes also have upstream open reading frame but VEGF's is a bit different, because it doesn't inhibit the ribosome from encountering the mRNA but instead promotes the AUG start site of the CUG. Upstream open reading frames are used to help regulate the level of expression under normal conditions and times of stress; in the case of VEGF this moiety also contributes to isoform expression. Furthermore miRNA and riboswitches are used in regulating translation, these processes have both be described by classmates on this page. The last form of regulation is G-quadraplex formation. It has been suggested that regions of the mRNA can form these species but how this contributes to the regulation of VEGF under different conditions is still unknown 4. Importance Angiogenesis is a critical event to regulate because uncontrolled blood vessel growth can damage tissues and promote tumors. One of the major players in the neovasculargenesis is VEGF. VEGF is a common target in treating cancers, stroke, and chronic non-healing wounds1,2. It is important that a proper equilibrium of VEGF is maintained because it is a necessary gene, and even a single copy knockdown is lethal 2,4. The tight regulation at every possible step: transcription, post-transcription, and translation, highlight this importance 3,4,7. References 1 Johnson, K.E., and Wilgus, T.A. 2013. Wound healing society. PMID:25302139 2 Talwar, T. and Srivastava, M.V. 2014. Ann Indian Acad Neurol. 17:1-6. PMID:24753650 3 Levy, A.P, Levy, N.S., and Goldberg, M.A. 1996. J.Biol. Chem. 271:25492-7 4 Arcondeguy, T. et al. 2013. Nucleic Acids Research. 41:7997-8010 PMID:23851566 5 Seneger, D.R., et al. 1983. Science. 219:983-5 PMID:6823562 6 Leung D.W., et al. 1989 Science. 246:1306-9. PMID:247998 7 Ahluwalia, A. and Tarnawski, A.S. 2012. Curr Med Chem 19:90-7. PMID:22300081 8 Kondo, K. and Kaelin, W.G. 2001. Exp Cell Res. 264:117-25. PMID:11237528